This invention relates to permanent magnets and more particularly to methods for making anisotropic permanent magnets of manganese-aluminum-carbon (Mn-Al-C) alloys.
Previously known Mn-Al alloy magnets consisting of Mn 60 .about.75 weight % (hereinafter referred to simply as %) and the remainder Al are such that the ferromagnetic metastable .tau.-phase is obtained by a heat treatment, e.g. the cooling control method or the quenching-tempering method. The Mn-Al alloy magnets, however, possess magnetic properties which are low, i.e. in the order of (BH)max=0.5.times.10.sup.6 G.Oe.
Since then, a method has been proposed for improving the magnetic properties of Mn-Al alloy magnets by applying a high degree of cold-working, i.e. swaging to the alloy to render it anisotropic. It is known that rod shaped Mn-Al alloy magnets in the ferromagnetic phase are sealed in non-magnetic stainless steel pipes, and while being held in said pipes, are subjected to swaging to a degree of 85.about.95%. This method is capable of producing an anisotropic permanent magnet possessing magnetic properties in the order of (BH) max.apprxeq.3.5.times.10.sup.6 G.Oe in the direction of preferred magnetization, i.e., the axial direction of the rod. Because Mn-Al alloy magnets are intermetallic compounds having very hard and brittle mechanical properties, however, even a cold-working of less than 1% causes cracks or fractures in the alloys.
On the other hand, since the degree of anisotropization is dependent upon the degree of cold working, it is necessary to cold-work the alloy to a high degree, normally higher than 80%, in order to achieve satisfactory magnetic properties and in order to be able to conduct such cold-working step, the cold-working operation must be conducted while the alloy is sealed in a non-magnetic stainless steel pipe.
An anisotropic permanent magnet obtained by using the above method is complicated in that the Mn-Al alloy inside the pipe must be finely pulverized into powder, and, moreover, it is difficult to obtain rods of uniform cross-section. The method is therefore costly and of little practical value.
In order to overcome the above difficulties, a method has been proposed for obtaining a rod shaped anisotropic Mn-Al alloy magnet by subjecting the Mn-Al alloy magnet to hydrostatic extrusion at a temperature below 200.degree. C, but the magnetic properties of such alloys is low, being in the order of (BH) max=2.5.about.3.6.times.10.sup.6 G.Oe in the direction of preferred magnetization. This method also required a very intricate hydrostatic extrusion operation and is again a very impractical method.
On the other hand, Mn-Al-C alloy magnets are isotropic permanent magnets in bulk shape excelling in magnetic properties, stability, weathering resistance and mechanical strength, and are disclosed in U.S. Pat. No. 3,661,567. Thus, according to U.S. Pat. No. 3,661,567, these alloys may be multicomponent alloys containing impurities or additives other than Mn, Al, and C, but should contain Mn, Al, and C as indispensable components. Isotropic permanent magnets with excellent magnetic properties, i.e. better than (BH)max=1.0.times.10.sup.6 G.Oe, while in an isotropic state (which is twice as high as the magnetic properties of isotropic Mn-Al alloy magnets) are manufactured by the quenching-tempering method with the component ratio of Mn, Al, and C in these multi-component alloys falling within the following range:
Mn 69.5.about.73.0 % PA1 Al 26.4.about.29.5 % PA1 C 0.6.about.(1/3 mn - 22.2) % PA1 Mn 68.0.about.73.0% PA1 C (1/10 mn-6.6.about.(1/3 Mn-22.2)% PA1 Al remainder
Subsequently, methods of warm plastic deformation of Mn-Al-C alloys have been devised to improve their magnetic properties. According to co-pending U.S. Patent Application, Ser. No. 491,498, now U.S. Pat. No. 3,976,519, issue date Aug. 24, 1976, said warm plastic deformation in the temperature range of 530.degree. C.about.830.degree. C provides Mn-Al-C alloy magnets of both single crystals and polycrystals with striking improvement in magnetic properties. In the compositional range wherein Mn is 68.0.about.73.0%, C is (1/10 Mn-6.6).about.(1/3Mn-22.2)% and wherein the remainder Al, there is produced a permanent magnet with magnetic properties such that the (BH)max is above 4.8.times.10.sup.6 G.Oe up to about 9.2.times.10.sup.6 G.Oe is its bulk state.
While the above process provides single crystal magnets having a (BH)max of up to about 9.2.times.10.sup.6 G.Oe, polycrystalline Mn-Al-c alloy magnets with the (BH)max above 4.8.times.10.sup.6 G.Oe and up to about 7.8.times.10.sup.6 G.Oe may also be obtained by the warm plastic deformation, e.g. warm extrusion. Compared with the rather complicated procedure for making single-crystal Mn-Al-C alloy magnets, the method of making polycrystalline Mn-Al-C alloy magnets by warm plastic deformation is much simpler and accordingly, is of high industrial value.
Furthermore, aforementioned co-pending application, Ser. No. 491,498 indicates that a remarkable improvement in magnetic properties is realized from the above-described plastic deformation and highly increased degree of anisotropization and that this is a new phenomenon based on a mechanism peculiar to the Mn-Al-C alloy magnets. This application further indicates that C is an indispensable component element. For example, in the case of Mn-Al alloy magnets, it was confirmed that slight plasticity appeared above 580.degree. C, but that by the working above 530.degree. C, no improvement in magnetic properties was recognized at all, rather, the magnetic properties were greatly degraded.
As disclosed in said co-pending application, Ser. No. 491,498, the Mn-Al-C alloys are magnetically directionalized by use of warm plastic deformation in a restricted condition, in which the warm plastic deforming should be applied in a specific direction in the temperature range of 530.degree..about.830.degree. C in order to cause the sliding of the plane of atoms in a specific direction. The magnetic directionalization can be made by said sliding of plane of atoms from any phases containing carbon as an indispensable component element, i.e., the close-packed hexagonal .epsilon.-phase, orthorhombic .epsilon.'-phase, face-centered tetragonal .tau.-phase, or a plurality of said phases. The .epsilon.' is a newly found intermediate phase in the .epsilon..revreaction..tau. transformation, which is represented by B 19-type structure (lattice constants, a=4.371 A, b=2.758 A and C=4.582 A).
The experimental studies on the Mn-Al-C single crystals have made it clear that only the restricted directional deformation or to be more precise, the direction of the compression to be restricted within the specific directional range, crystallographically causes the atom sliding so that the formation of the objective directional ferromagnetic .tau. phase is available. Said directional deformation should be one that causes the sliding of the plane of atoms in the (0001) .sub..epsilon. phase to the [1100].sub..epsilon. direction of the .epsilon. phase, in the same corresponding (100) .sub..epsilon..spsb.' plane to the [001].sub..epsilon..spsb.' direction of the .epsilon.' phase, and the same corresponding (111) .sub..tau. plane to the [112].tau. direction of the .tau.-phase. With respect to the compression direction relative to the .tau.-phase crystal orientation after deformation, compression in the direction nearly perpendicular to the [001].tau. axis of the obtained .tau.-phase single crystal after deformation, which axis is the objective direction of easy magnetization, facilitates sliding of the plane of atoms in every phase. When compressed under such conditions, deformation anisotropy was notable, i.e., remarkable shrinkage in the direction of the compression was observed, and notable elongation was recognized in the only one direction perpendicular to the compression direction. The direction where notable elongation was recognized is nearly the same as the direction of easy magnetization of the obtained .tau.-phase single crystal. Accordingly, a magnetically anisotropic single crystalline magnet is the uni-directional .tau.-phase was obtained from any of the phases mentioned above.
Thus, with regard to the single crystalline Mn-Al-C alloys, the necessity of the specific directional deformation was recognized for making magnetically anisotropic permanent magnets.
Regarding the polycrystalline Mn-Al-C alloys, said co-pending application, Ser. No. 491,498 also has indicated that as the alloys were upset in said temperature range, anisotropic permanent magnets with the direction of preferred magnetization being in the direction of diameter which was perpendicular to the direction of compression, were obtained. And, when the alloys were extruded, it was made clear that the direction of preferred magnetization was in the direction of extrusion.
However, behavior of the Mn-Al-C polycrystalline alloys under deformation in the warm extrusion process in very complicated because of effects such as diversification in orientation of minute crystals, grain boundaries and friction, etc. Because of the above difficulties, the influence of profiles of the extrusion dies bounding the alloys in order to cause plastic deformation therethrough is unclear. In particular, the contribution of the die angle to the formation of magnetically directionalized .tau.-phase polycrystalline alloys, has not been clarified.